1. Field of the Invention
The present invention relates to a printhead, a head cartridge, and a printing apparatus employing either of the printhead and the head cartridge, suitable for use in inkjet printing in which ink droplets are discharged.
2. Description of the Related Art
An inkjet printing apparatus is configured so as to print information on a print medium by discharging printing ink from a plurality of fine nozzles of a printhead according to a print signal. An inkjet printing apparatus has advantages such as being capable of non-contact printing to a print medium such as print paper, easy conversion to color, and being abundantly quiet.
FIG. 1 is a block diagram showing a circuit configuration in a conventional printhead, and an example timing chart of signals transmitted to the printhead from a printing apparatus.
A DATA signal 101 (digital signal: image data) is serially input to a shift register 105 in synchronization with the edges of a CLK signal 102 (clock signal). The shift register 105 temporarily stores data that has been input, the data corresponding to heaters.
Next, when an LT signal 103 (latch signal) that is input to a latch circuit 106 goes low, the data stored in the shift register 105 is latched in the latch circuit 106. When the LT signal 103 goes high, the data latched in the latch circuit 106 is held.
When an HE signal 104 (heat enable signal) is input to a drive circuit 107 in a state with data being held, electric current flows to a heater corresponding to the data according to time, at a timing when the HE signal 104 goes low.
Printing is performed by repeatedly performing the above sort of data transfer and printhead driving.
On the other hand, at present, increased performance such as faster speed is sought for inkjet printing apparatuses. Thus, as a means for increasing printing speed, there are methods wherein the cycle in which ink is discharged from the nozzles is shortened. However, there are problems such as the fact that due to shortening that cycle, many signals are transmitted with high frequency, and thus there is an effect from radiated electromagnetic waves.
Also, in a thermal inkjet printing apparatus, in addition to a signal, a high current that is allowed to flow to a heater flows to the printhead by being transmitted through a long flexible cable or the like from the main body of the printing apparatus, and noise generated by this current interferes with the signal. There is a risk this will lead to malfunction of the head. Thus, it is necessary for a thermal inkjet printing apparatus to be robust with respect to noise caused by this current.
Consequently, a differential input circuit has been disclosed as one way of dealing with these problems (see Japanese Patent Publication Laid-open No. 10-166583).
In Japanese Patent Publication Laid-open No. 10-166583, two signals are input to the differential input circuit for one type of signal. Here, as an example of data transfer to the differential input circuit, a timing chart is shown in FIG. 2. Here, two signals (double-ended signals) CLK+ and CLK−, and DATA+ and DATA−, are provided for each of the CLK signal and the DATA signal, which require transfer at high frequency.
Because the double-ended signals use the differential of the ± signals, they can be transferred as signals with a smaller voltage amplitude than in the case of a conventional single-ended signal. Thus the electromagnetic radiation of those double-ended signals is less than when transmitting with a single-ended signal. Also, because these signals pass through a flexible cable or the like adjacent to each other, electromagnetic noise generated from the current of each signal is cancelled by the other, thus suppressing interference with the other signal.
Thus, electromagnetic radiation during high frequency signal transmission, which was a problem for accelerating printing speed, can be suppressed by adopting differential data transfer.
Also, differential data transfer is very robust with respect to external noise. Because a double-ended signal uses the differential of the ± signals, even if external noise is applied, the differential does not change provided that the same noise is applied to both the + and − signals, so the effect of noise can be ignored.
Thus, robustness with respect to noise caused by high current flowing to a heater is insured, so it is possible to prevent malfunction of the head.
However, because it is necessary with differential data transfer to use two signal lines to transfer signals conventionally transferred with one signal line, such a configuration leads to increased size of the apparatus and also increased costs, due to an increased number of input pads and an accompanying increase in wiring.
So, there is a desire to reduce the number of input pads, suppressing an increase in the number of input pads and an accompanying increased size and cost of the apparatus, while making use of the advantages of employing a differential input circuit.